Microplastics Inhibit a Hermit Crab's Ability to Choose Shell

CC BY-SA 2.0. Timo Newton-Syms

An Irish study found that pollution affects cognition and they're unable to detect an ideal shell when they see one.

Hermit crabs are pros when it comes to moving house. As soon as they outgrow a shell, they scope out new options and upgrade to a bigger size. They have this down to a fine art, with entire groups of crabs lining up from biggest to smallest and waiting till the right moment to exit their too-small shell and beeline for the bigger one. Needless to say, this behavior is essential to their survival. Crabs are vulnerable without their shells and they're always growing.

But plastic debris is wreaking havoc with their ability to choose new shells, and it goes beyond mistaking plastic containers for shells, which Melissa wrote about several months ago. New research from Queen's University in Belfast, Ireland, has discovered that exposure to microplastic particles in the water actually inhibits a crab's ability to assess the potential of a new shell. As study co-author Dr. Gareth Arnott explained, "The striking thing in this study was when [we offered them a better shell], lots of the crabs that had been exposed to the microplastics didn’t make the optimal decision to take [it]."

The study, published in Biology Letters, describes the research process. Two groups of female crabs were placed in two separate tanks, one with 29 and one with 35. Both tanks were filled with seawater and seaweed, but one contained 4mm-diameter polyethylene beads. The crabs remained in the water for five days, then were removed, taken out of their shells, and given new shells to move into – except that these were not shells that the crabs would've chosen themselves, "about half the ideal weight for each crab." Two hours later the crabs were presented with new shells of suitable size. The researchers were surprised by their observations:

"The team found that 25 of the crabs who had not been exposed to the microplastics explored the optimal-sized shells, with 21 of the crabs – 60 percent – taking up residence in them. By contrast, crabs that had been exposed to microplastics took longer to begin such exploration and far fewer did so: just 10 made contact with the optimal-sized shells and only nine – 31 percent of the group – moved home."

This suggests that exposure to plastic particles changes the way in which crabs perceive their shells; in other words, pollution is affecting cognition, which is deeply troubling, considering the extent of plastic pollution on beaches around the world and that being capable of keen assessment is an essential survival skill for hermit crabs.

Arnott said, "We hypothesize that either some aspect of the polyethylene is getting into them to affect their decision making, or else it is an indirect effect that the presence of the plastic in the tank might be influencing their feeding behavior, for example."

Further research will delve into the actual mechanism at play, whether other crab species are affected, if all microplastic types have the same effect, and whether this sad interaction is playing out in the wild as it did in the laboratory. And in case you were wondering, all of the crabs used in this study were returned to the beach in Ireland unharmed.